Grinding an Edge Ends Up Distorting Silhouette?

[Tom MacNucator]

Good Afternoon everyone,

I've been working on a batch of Puukkos (a Finnish general purpose knife) for practice and have encountered a problem while grinding the bevel on the cutting edge.I'm finding that the grinding is actually distorting the curve leading to the tip. I'm afraid this may be due to over-grinding on my part (grinding until it's sharp), or maybe my setup (blade champed to the table, using an angle grinder for rough work and then a handheld belt sander for refining the bevel and cleanup).

The only other thing I can think of is that I may be doing things out of order. In what order do you guys put edges on tools?

I've attached an image to try to explain my problem. The top is what I'm trying to achieve, the bottom is what is happening.

Thank you.

[TwistedCustoms]

The handheld tools are probably the cause. If you're using a 4x24 or 3x19 hand held sander try flipping it upside down and mounting the forward hand grip in a vise. Then you can lock the trigger in the run position and have both hands free to hold the work on the belt. Of course that type sander wasn't designed to be used that way, you will eventually get metal dust in the motor and shorten the life but in a pinch....   Anyway, investing in a belt grinder designed with knife makers in mind will move your work ahead faster than improvising and jury rigging. You can always make a file jig and hand file the bevels too. Scandi, flat grinds are quick and easy with a bevel block but that requires having a tool rest mounted 90 degrees to the platten. It would be tricky to set up that type work rest on a hand held sander but it could be done.

[rockstar.esq]

It looks like you're struggling to keep the bevel length the same as the bevel turns up to the tip.  If you're sliding the stock perpendicular to the belt/wheel/whatever, you're going to end up with the bottom example every time.  The tip portion must rotate across the abrasive surface.  So if you were grinding edge up handle to the left you'd start near the handle sliding left, then you'd need to rotate the handle down in correlation to the tip transition.  The "stroke" needs to end with your tip pointing up.

Instead of using a flat plate to rest your blade on, consider using a stepped dowel mounted central and perpendicular to your abrasive surface.  The step in the dowel keeps the blade from kicking out of the support, but the round surface allows you to maintain support while pivoting the stock.

You could mark the blade spine on each side with a magic marker to indicate where to start your pivot.

 

[Kozzy]

Think of the area of contact between the belt and the knife.  On the long straight section, the contact is basically across the whole width of the belt.  As you turn the corner, the contact reduces to a very small  portion of the belt--essentially almost a line that's not more than about half a pinky's width at the most.  That makes it really easy to lose control of all the issues which maintain the shape from pressure to movement.  If you maintain the same apparent pressure (like just the weight of the sander), that pressure is over a MUCH smaller area and it will start hogging off too much material.  Hesitate a fraction of a second and that half-pinky's width tries to flatten out wider and you take away the nice rounded section.  As things progress, the radius will get larger and larger due to the lack of control.  

It's almost impossible to control a hand held sander in those conditions.  Fixed belt is the way to go.  Even a low-end knife belt sander (check pawn shops) would be an order of magnitude better in ability to control the grind.  Even with a proper knife belt sander, you have to maintain good control on the curves.

Twisted has one solution that allows you to control the blank instead of the sander--which will help a lot.  I've used twisted's method and clamped the belt sander in a wooden woodworker's handscrew style clamp.  It makes the system pretty stable on the workbench and it's hard to break the sander by over-clamping (vs something like a vice).  However, that was in the stone age when belt sanders were actually made of metal so YMMV.

If you're stuck with the handheld you have for now, you also might try going with a less aggressive grit on the curved sections to increase control.  Takes longer and can generate more heat but it might help you tweak the form rather than hog off too much in the blink of an eye.  

[Frosty]

I don't run across many people who get it when I try to show them how to use a sharpening stone and the technique applies to grinders, sanders, etc.

I visualize the blade on a plane that intersects the stone. When I move the blade I maintain it's orientation on the plane not in my hand. Most people make the tips really shallow bevels and lengthen the profile by maintaining the angle of their hand. You hand isn't the important thing, it's the blade.

If you look at any sharpening guide, jig, etc. tool you'll see it maintains the angle on a plane.

Frosty The Lucky.